Worldwide, there were over 6,000 new HIV infections a day in 2010 and almost half were among women. Injectable contraceptives have been associated with a 2-4 fold increased risk of HIV acquisition in some observational studies. Yet, these injectable progestin's, like Depo-Provera (medroxyprogesterone acetate [MPA]), are the second most common contraceptive method used by women worldwide. Animal data support the increased HIV risk. Prospective clinical evidence is lacking. If injectable contraceptives increase HIV susceptibility, and if heterosexual women at high-risk for HIV infection desire to continue injectable contraception use, effective HIV prevention methods such as pre-exposure prophylaxis (PrEP) become even more important. For women at high risk of HIV infection, PrEP with daily oral tenofovir disoproxil fumarate/emtricitabine (TDF/FTC) can substantially reduce the risk of HIV acquisition. In July 2012, the U.S. Food and Drug Administration approved TDF/FTC for PrEP and emphasized that effectiveness is strongly correlated with drug concentrations. Individuals with lower than expected drug concentrations had higher HIV seroconversion rates. While adherence to daily TDF/FTC is thought to explain the differences in plasma concentrations, our preliminary data indicate that sex steroid hormones may also contribute to lower antiretroviral concentrations and increase HIV susceptibility. Given these findings, we hypothesize that the injectable contraceptive, MPA, increases HIV susceptibility through inducing immune activation and increasing HIV co-receptor expression on genital tract target cells. Further, we hypothesize that MPA decreases effective TDF/FTC concentrations through increased renal clearance secondary to increased glomerular filtration and/or up-regulation of progesterone-sensitive drug transporters on CD4+T and kidney cells, as well as alteration of drug phosphorylation and activation in CD4+T cells. Our specific aims are to: (1) determine the effect of MPA on HIV susceptibility in blood, cervical tissue, and cervicovaginal fluid; and (2) quantify the impact of MPA on TDF/FTC in vivo pharmacokinetics and, secondarily, in vitro pharmacodynamics in blood, cervical tissue, and cervicovaginal fluid. There is an urgent need to address this dilemma regarding injectable contraceptives in women at risk for HIV infection. Does the use of injectable MPA, a highly effective and inexpensive birth control option, increase HIV acquisition risk and diminish the HIV protective effects of TDF/FTC PrEP? This information is needed to inform women's contraceptive choice, especially among those at high risk of HIV.